1. Field of the Invention
This invention relates to a semiconductor memory device, and more specifically relates to an electrically rewritable and non-volatile semiconductor memory device (EEPROM).
2. Description of the Related Art
NAND-type flash memory is known as one of EEPROMs. There is increased a demand for NAND-type flash memories in accordance with increasing of the demand for mobile devices, which deal with large capacitive data such as a still or moving image. Being formed of NAND strings (NAND cell units), each of which has multiple memory cells connected in series, a NAND-type flash memory has such features that the unit cell area is small; and it is easy to make the capacity large. While, there is such a drawback that the NAND-type flash memory is not suitable for a high-speed random access-use because the cell current is small.
In consideration of this, the data transmission rate is made high in such a way that data are read out to a page buffer, and then serially output, thereby resulting in that NAND-type flash memory is made adaptable to a high-speed system via a buffer DRAM and the like.
A sense amplifier usually used in the NAND-type flash memory is a voltage detecting type one, which precharges a bit line, and then detects the bit line voltage after discharging it with a cell current for a certain time (for example, refer to JP-A-2004-118940).
By contrast, there has already been provided a differential sense amplifier of a current sensing type, which is possible to sense a small cell current at a high rate (for example, refer to JP-A-2005-285161). By use of this sensing scheme, in which a pair of bit lines becomes differential inputs, it is also possible to avoid interferences due to capacitances between bit lines.